1. Field of the Invention
The present invention relates to refrigeration apparatuses that attain an ultralow temperature of −80° C. or the like and in particular relates to a refrigeration apparatus that uses a refrigerant composite material containing carbon dioxide (R744).
2. Description of the Related Art
Conventionally, for example, a refrigerant having a low boiling point, such as ethane (R170) having a boiling point of −88.8° C., R508A having a boiling point of −85.7° C. (azeotropic mixture of 39 mass % trifluoromethane (R23) and 61 mass % hexafluoroethane (R116)), and R508B having a boiling point of −86.9° C. (azeotropic mixture of 46 mass % trifluoromethane (R23) and 54 mass % hexafluoroethane (R116)), is used in a refrigeration apparatus capable of cooling its interior to an ultralow temperature of −80° C. or the like (e.g., refer to patent document 1).
In addition, to reduce the global-warming potential (hereinafter, referred to as GWP) and the inflammability, it is being proposed that carbon dioxide (R744, GWP=1) be mixed with the aforementioned primary refrigerant. Carbon dioxide (R744) has high thermal conductivity, and mixing carbon dioxide (R744) produces such effects as an increase in the density of the refrigerant sucked into a compressor and an increase in the circulation amount. Thus, an improvement in the refrigeration performance can be expected from such mixing with the aforementioned primary refrigerant.
In addition, in this type of refrigeration apparatus, the performance is improved by constituting a double pipe by a suction pipe through which a refrigerant that returns from a final-stage evaporator to a compressor passes and a capillary tube through which a refrigerant travels toward the evaporator, and by allowing the refrigerants to exchange heat therebetween (e.g., refer to patent document 2).
[patent document 1] Japanese Patent No. 3244296
[patent document 2] JP2011-112351
The boiling point of carbon dioxide (R744) is −78.4° C., which is high as compared to that of ethane (R170) or the like serving as a primary refrigerant, and carbon dioxide (R744) is less likely to evaporate even in a final evaporator. Thus, the refrigerant exiting from the evaporator contains a very high proportion of carbon dioxide (R744) and is at an ultralow temperature of −80° C. or the like. Meanwhile, pressure loss is likely to occur at the aforementioned double pipe portion, leading to a situation in which carbon dioxide (R744) is solidified at this portion and turns into dry ice, which clogs up a pipe in a refrigerant circuit.
Thus, there has been a problem in that this dry ice prevents the refrigerant from circulating in the refrigerant circuit, leading to a sudden rise in the temperature inside the refrigeration apparatus.